Background: My weathermatic SLW10 rain sensor stopped working last year after a good season of helping contain the water bill. The sensor system consists of a hub (SL-HUB) that is placed inside the irrigation controller that communicates with the weather / wind / etc. sensor over two wires. The sensor platform is a 9V-block battery-powered unit. Based on my research, I ordered a replacement SL-HUB, and it works like a charm. Being curious re: the failure mode of the original unit, I decided to dissect the old one. Here is what I think I found and what has me scratching my head.

The SL-HUB appears to be powered via the same 24VAC bus as the rest of the weathermatic unit (have to confirm this via measurements). As you can see in the circuit I traced below, the incoming power runs through a half-wave rectifier before it gets to fill a 100uF 50V cap. Then it is run through a 12kOhm resistor before being regulated down to 5V with two de-coupling caps to keep things smooth. The LM7805CZ is made by WST and the maximum allowable voltage is 40V per its data sheet. I have measured the voltage coming into the weathermatic controller to be as high as 27.3VACrms. In other words, the peak voltage is around 38V. Doesn't this seem a bit close to the edge re: the maximum ratings mentioned on the datasheet?

Additionally, is it just me, or is this a weird way to deal with 24VAC incoming power to make 5VDC? All the circuit appears to be powering is three transistors and a DS1487 RS485 transceiver. Perhaps even a drop-cap supply could have worked

Another nice touch is the diode that was hand-soldered from one of the interface pins to the 5VDC output pin of the 7805. I presume this is to prevent the voltage on said pin to go higher than 5VDC?

The hand soldered diode (i.e. not the SMD diode D1 that is part of the power supply circuit) appears to run from a signal line on the pin header (which is how the SL-HUB interfaces with the irrigation controller to the 5VDC pin on the LM7805. Given the crude solder job, the lack of a printed circuit, header holes, etc., and the lack of any kind of designated spot for the thing, my guess is that this diode is something they figured out they needed after a bazillion boards had been already printed. My guess regarding its intended duty would be signal protection, i.e. it prevents signals from exceeding ~5VDC.

I have also just suffered a sudden failure of a Weathermatic SLW10 during its service life less than 1 year in. The fault is recorded as a communications failure, and what I notice is that the LED inside the weatherstation fails to flash any longer when the reset button is pressed.

You said that you ordered a new SL-HUB "based on your research". Can you say what that was, please? Currently Weathermatic's response is that it's covered under warranty, which would have me take the whole station and hub, box it up, ship at my expense back to the Internet provider I bought it from, hope that they agree that it is broken (or they keep the station !!!) and then have a new one shipped to me (also at my expense). This is what a warranty service comes to these days, apparently.

I suspect the cost of a new hub would be less than all that (and much less risky). So, how did you know that only a new hub was probably needed?

...I suspect the cost of a new hub would be less than all that (and much less risky). So, how did you know that only a new hub was probably needed?

I did it on the basis of the weather station behaving as it should, i.e. multiple flashes of the diagnostic LED in the right sequence made me believe the unit was OK, and that the receiver was at fault. So I bought a new SL-HUB on ebay for $29 and tried it out. Worked for about a week and then the unit on the roof started making error codes that are undocumented. So I gave up and replaced the SLW10 with new wireless version. If it wasn't the voltage that killed the unit, nearby lightning may have done it.

However, based on my trace of the unit, my best guess is that the power supply on the unit inside the controller cannot handle my local conditions. My primary voltage is always around 125VAC, and by the time it lands inside the controller, the nominal 24VACrms turns into 27VACrms.While the rest of the controller seems to handle that OK, the plug in module with its minimal power supply does not seem to. Interestingly, CUI now offers a tiny drop-in replacement for 7805's with a very wide input voltage range. Admittedly, they are not cheap (about $8), but it may be my best bet to get a power supply that can handle the hazards of my local conditions - up to 72V with the 78w-500.

That circuit cannot possibly work with the component values you have shown. I suggest you re-check the values. In particular, I think the 12k resistor must (or should) have a much lower value. Can you read the colour code? Does it show any signs of charring?

Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

That makes more sense. There will always be at least 5mA (plus the quiescent current of the 7805) flowing through the 1.2k resistor, because of the 1k load resistor at the 7805 output. Therefore there will always be at least 6V dropped across it. So the input voltage would have to exceed 46v before the 40v regulator limit is exceeded.

Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.